Extracted ion chromatogram (XIC) of the phosphopeptide containing the autophosphorylated Tyr-416 of Src and Src (K295M) in the presence and absence of Csk. A-D Src (K295M) mutant alone (A), Src (K295M) with Csk (B), Src alone (C) and Src with Csk (D) were allowed to undergo phosphorylation in vitro. Peptides are identified by Mascot in accordance of their retention time. Retention time and mass error (ppm) are shown. E. Isotope dot product (idotp) comparing the observed and theoretical distribution of the precursor isotope with 1.0 being an optimal match. (TIFF 1102Â kb)